Generally, a pulse tube refrigerator is used as a cooling device, which is built into an apparatus requiring an extremely-low temperature such as, for example, a nuclear magnetic resonance diagnostic apparatus (MRI), etc.
In a pulse tube refrigerator, an operation of causing a coolant gas (for example, helium gas), which is an operating fluid compressed by a compressor, to flow into a pulse tube and an operation of causing the operating fluid to flow out of the pulse tube and a regenerator regenerator and collecting the operating fluid are performed. By repeating these operations alternately, low-temperature ends of the regenerator and the pulse tube can be at an extremely-low temperature. A cooling object is cooled by bringing the cooling object into thermal contact with the low-temperature ends.
From among pulse tube refrigerators, especially a multivalve-type pulse tube refrigerator has a high cooling efficiency and application in various fields is expected.
In a multivalve-type pulse tube refrigerator, a coolant gas is caused to flow to an appropriate part and in an appropriate direction at a predetermined timing. In order to do so, a plurality of valves must be mutually related and the valves must be opened and closed at predetermined timings. For example, PCT Japanese Translation Patent Publication No. 2007-522431 discloses a rotary valve used as a member integrating a plurality of valve functions.
The rotary valve includes a rotatable rotary disk and a seat, which is stationary. A plurality of holes (grooves), which are communicated with a high-pressure side and a low-pressure side of a compressor, are opened in a generally circular flat surface (face) of the rotary disk. Additionally, a plurality of ports, which are communicated with a regenerator and a pulse tube, are opened in a generally circular flat surface (face) of the stationary seat. Accordingly, when the rotary disk is rotated while pressing the face of the stationary seat onto the face of the rotary disk, and when the relative positions of the faces (more specifically, relative positions of the holes and the ports) are set in a first predetermined positional relationship, a supply flow path of a high-pressure coolant gas is formed from the compressor to the regenerator and/or the pulse tube. On the other hand, when the relative positions of the faces (more specifically, relative positions of the holes and the ports) are set in a second predetermined positional relationship, an exhaust flow path of a low-pressure coolant gas from the regenerator and/or the pulse tube to the compressor. As mentioned above, the rotary valve can alternately change the flow path of the coolant gas by rotating the rotary disk.
In a generally used rotary valve such as disclosed in the above-mentioned PCT Japanese Translation Patent Publication No. 2007-522431, a plurality of first ports, a second port, a third port, a fourth port and a fifth port are provided in the face of the stationary seat. The first ports are used to introduce a high-pressure coolant gas into the regenerator. The second port is used to introduce the high-pressure coolant gas into a first stage pulse tube. The third port is used to exhaust a low-temperature coolant gas from the first stage pulse tube. The fourth port is used to introduce a high-pressure coolant gas into a second stage pulse tube. The fifth port is used to exhaust a low-pressure coolant gas from the second stage pulse tube. The first ports are positioned along a first circumference (track) equidistant (that is, a radius) from the center of the face of the stationary seat. The second port and the fourth port are positioned on a second circumference (track) equidistant (that is, a radius) from the center of the face of the stationary seat. The third port and the fifth port are positioned on a third circumference (track) equidistant (that is, a radius) from the center of the face of the stationary seat.
In other words, the face of the stationary seat must have three different circumferences for the first port, for the second and fourth ports and for the third and fifth ports. For example, the length of the first port for the regenerator (an overall length in a radial direction of the face) is about 10 mm, and the distance (radius) of the first track from the center of the face is about 20 mm.
However, according to the above-mentioned structure, it is difficult to reduce the diameter of the face of the stationary seat to a diameter smaller than a diameter which can include the three tracks. Thus, the diameter of the faces of the stationary seat and the rotary disk are necessarily large, thereby increasing a size of the rotary valve. If the size of the rotary valve is large, a position of the rotary valve in the pulse tube refrigerator is limited, and a torque needed to rotate the rotary disk is increased. Moreover, if the rotary valve is large, an amount of abrasion generated due to wear of the faces is increased.